Happens with Alfred Teves (Ate) & Brembo all the time with modern calipers. e.g. Mercedes. They are the 2 major suppliers & the problem is as old as disc brakes themselves. This does not fix the locking problem. That is usually Hill holder or Servo.

https://www.jaguarforums.com/forum/m...ds-etc-211186/

 

Servos don't like fancy brake fluids like silicone. DOT 4+ max should be used. (elastomer compatibility issue)

 

I'll wander off topic here as I don't understand Huey and Glyn's explanations of pad retraction. I can imagine the seals pull the pad back as the small elastic deformation they endure during braking is relieved. In addition, the small imperfections and deflexions of the disc-hub system will knock the pistons back a little. Once there's the smallest local clearance, I can imagine the aerodynamic boundary layer on the disc being dragged into the space and pushing the pad back further - kind of like classic slipper lubrication theory. And I guess that any remaining asperities on the surfaces might be decapitated by mutual interference. What I can't figure is friction that by definition (?) acts parallel to a surface pushes two surfaces apart. My guess is that all this applies as much to old Dunlop brakes as modern designs as Dunlop's retraction mechanism tends to give up or is removed.

I have a vague memory (so I may well be wrong) that GM started developing their own disc brakes possibly for the Corvette. For some reason, they didn't want to use a brake booster (though it seems out of character for a company that was usually power assisted everything). They needed maximum mechanical advantage from pedal to pad without excessive pedal stroke. This required nominally zero pad retraction, so they set it to zero and decided to tolerate some power loss to friction. It turned out to work fine.

​​​​​Apologies if I've got things totally wrong.

 

Thank you, all! Now, what are the safest points to jack up the front wheels as a pair and the back wheels as a pair? I see references to jacking under the center of the front cross member in the front - is that the piece in front of the wheels? In the rear, I guess the differential works?

@ Peter - That’s two really helpful points. It hadn’t occurred to me that I could test this stationary. I’ll jack up the front and test there - that will simultaneously exonerate the anti roll mechanism and confirm the servo, and I’ll feel more free to really press hard when I know I’m not burning brakes in motion.

@Bruce - understood. I think my question about the breather was directed to a comment that Peter had made. I read through the document you sent with some care this morning and finally understand the mechanism. That will help immensely. I’ll take some more time to diagnose the unit I have since it seems I might just get another of the same. It saddens me that the PO may have let JK dispose of (or keep) the original core when a rebuild was an option.

@Glyn - From the records I have, this car has DOT4 installed, so I don’t think the DOT5 issues are at play. I’m always tempted to switch to silicone to avoid the paint issues I’ve experienced on other cars with mishaps around the master cylinder but never have yet.

 

Geoff ~ stick to that practice with our cars. If you get brake fluid on paint just flush copiously with water. Brake fluid is hygroscopic & will flush away and do no damage as long as not left in place on paint for a long time.

Peter ~ my explanation of seal deformation & not sliding down the bore is spot on as Brembo will confirm. A condition known as stiction. If you constantly drive a car gently in town you will notice that as your pads wear so your pedal drops requiring more fluid to bring the pad back into contact with the disc/rotor & thus a lower pedal. That is stiction (a shock absorber term used by Bembo). The seal deforms & does not slide down the bore. Making disc brakes in certain conditions not self adjusting i.e. caliper piston over return. My Merc is doing it now & is due for new front pads shortly at service. Brakes take at about a 3rd of pedal down/travel. With new pads correctly installed I will have a much higher pedal take to achieve braking. I have many satisfied customers/members at MBWorld that have found my comments to be absolutely accurate. Brembo makes all brakes for Mercs & agrees. It is commonly known in the braking/brake OEM industry. Do not push your pistons back into the caliper more than absolutely necessary to install the new pad or you could suffer the condition from day one.

Understand that some vehicles have the seal in a grove in the bore & others attached to the piston. Both practices exist. Both suffer the same condition.

 

I see now - I think the cross member is the more sturdy piece between the wheels. That's what I'm used to thinking of as front cross member, but I'd been seeing the piece just behind the bumper labeled that way. As long as I can get a jack far enough back, I'll stick with the cross member that is integral to the front suspension. I'll place a spare piece of wood underneath to help spread the load a bit until I can get the jack stands under the jacking points behind the front wheels.

Geoff

 

I just put the jack in the centre of the axle with a piece of hardwood, although my new one is rubber padded.
I put jack stands under the rear axle, or on the rear jacking points if one intends to remove the axle.

And about DOT 5, I am running that in my Jag and after an extensive brake restoration, it worked right out of the box, however there was and still is a problem.
The seal inside the servo that keeps the fluid out if the vacuum cylinder has failed, probably not compatible, I have since ordered some new ones and have had them soaking in DOT 5 for over a year and they are fine.
Eventually I will have to tear into the servo and find out what exactly went wrong.

The problem with the servo filling up with DOT 5 is eventually it will get sucked into the engine and destroy it, so I have not been driving it.

Just thought I would put this here when new members come across this thread.

I switched to DOT 5 because the slave cylinder leaked quite badly and got fluid everywhere, so bad that the accelerator pedal had rusted solid.
All the paint in that area, inside and out had to be redone.
More neglect than anything else I suppose, but when I stored it for the winder in a heated garage, the slave cylinder was not leaking.
When I took it apart it was a mess of white pasty aluminum and brake fluid.

The silicone fluid is not leaking out of the slave cylinder after the rebuild.

 

Glyn, We are describing the same phenomenon using different terminology. We are both saying that when the pad moves toward the disc, the piston may partly slide over the seal and partly deform it elastically. For small motion of the pad, it can be entirely accommodated by elastic deformation. When the pedal is released, the elastic deformation relaxes and pulls the pad back. Though the term striction is descriptive, it should be clear that it applies to the contact between the piston and the seal (and has nothing to do with friction between the pad and the disc/rotor).

Geoff, When you say cross member, I guess you mean the subframe. That's strong; the sort of cross member running across the car roughly under the radiator isn't suitable for jacking!

 

The whole problem exists that elastomer manufacturers do not label seals as silicone compatible or not. If they did we would have no issues with silicone brake fluid. Servo/booster manufacturers are notorious for not fitting slicone resistant seals. Why? ~ who knows.

 

Peter ~ agreed & the term stiction I'm using in the correct sense. I'm not suggesting it has anything to do with friction between the pad and the disc/rotor). I have never even implied that.

You stated "I don't understand [removed] Glyn's explanations of pad retraction." obviously with piston.

 

If you look back to what you quoted from Huey ...

 

The only online seller that came right out an admitted that all their aftermarket seals they sell are compatible with everything, is IntroCar, that's their "Prestige" brand.
That's good to hear about silicone brake fluid if it's just seal compatibility, there are at least 2 arguments other than compatibility that people argue about, but I'm not going to get into that here.

 

OK ~ my bad slightly. My mind was on one track. One that I explained clearly in following comments.
I also provided to Huey both links here & to MBWorld which I'm pretty sure were not read by your good self.
So I have duly looked back & find no fault on my behalf....

 

Appreciated. My argument is purely seal/elastomer compatibility. I know some of the other arguments & they tend to be nonsense & don't belong here.

 

Peter, yes that’s what I realized. I’d seen a few parts suppliers calling that bit under the radiator as a cross member and nothing else until I thought to check the suspension section. Glad I asked!!

 

jaguargeoff
Before committing yourself to a new power brake booster I think you should investigate the master cylinder. I will quote from my MK2 workshop manual

"When the brake pedal is in the "off' position it is necessary that the master cylinder piston is allowed to return to the fully extended position otherwise pressure may build up in the system causing the brakes to drag or remain on"
Jaguar also says "the pushrod clearance will give about 1/4 inch of free movement at the brake pedal pad and can be felt if the brake pedal is depressed gently by hand"
In my experience there are a couple of areas which can cause a problem.
Crud or rust in the rear of the master cylinder stopping it coming right back. Unlikely if it was properly overhauled BUT was it ?. I have found this on a couple of occasions
A broken brake return spring in the pedal box which leaves pressure on the master cylinder piston drive rod
Cheers

 

Thank you - I'm game. I checked the pedal and confirmed I have the intended 1/4" free movement. How do I check the rest? Access to the end of the push rod doesn't seem to be described in the workshop manual. Can I see it from inside the cabin by removing the panel in the footwell, or do I need to remove the MC entirely to check its range of motion?

I've been delayed in any more serious checks until I can move the car from its current temporary home in my garage down to a space in the shop. Right now there is "no room in the inn" but I'll get it cleaned out soon.

 

From your description of the free play, I believe the master cylinder is returning to the "fully extended" state and in that case, it is not the cause of your problem.
From that point it appears that the power brake booster is now the most likely cause.
In that case I would get the car to a large parking area and give the brakes and servo/booster a number of hard stops to the point of nearly locking up the wheels
That may free up the servo/booster system as it is taken to its maximum working pressures and component displacement. Its a bit brutal but worth a try.
If that fails, then unfortunately you need a servo/booster overhaul or replacement

 

Sometimes brutality is required with our old cars. Ask Peter about OD engagement ~ disengagement on an Autobahn I think. Well somewhere in Europe anyway.

 

That's how I discovered that, in spite of 100,000 miles, my old car could still reach the magic 120 m/h. The turbulence noise past the window frames was a bit intense.